A thermally insulating ling for the housing of a heater system has been proposed in which the heater space is provided with an insulating firebrick lining directly adjoining the outer wall of the housing. Such wall linings have however not proved to be suitable in all cases, because the thermal insulating effect is not sufficient to prevent the escape of the heat to the outside as is desired. The temperature of the products of combustion in the heater space normally obtaining of approximately 2000.degree. C. is such that the escape of heat may only be prevented to a limited extent so that the external wall of the housing of the heater system radiates heat so intensively that the use of Stirling engines is largely out of the question in small spaces in which workers have to be present.